PENETRATION AND BINDING OF EPIDERMAL GROWTH FACTOR-DEXTRAN CONJUGATESIN SPHEROIDS OF HUMAN GLIOMA ORIGIN

Targeting with toxic EGF-based conjugates against tumour cells with am plified EGF-receptors might be a possible approach towards improved th erapy of certain malignancies such as gliomas and squamous carcinomas. In this study, the penetration and binding of I-125 delivered by EGF- dextran conjugates were analysed in cultured spheroids applied as a tu mour nodule model. The spheroids consisted of human glioma cells, U-34 3MGaCl2:6, with large amounts of EGF-receptors. The penetration and bi nding patterns of I-125 delivered by I-125-EGF and I-125-dextran were analysed for comparision. The EGF-dextran associated I-125-activity sh owed a rather slow penetration but after some hours significant amount s of radioactivity had reached the deeper regions and good penetration was obtained within 5 hours. The penetration seemed somewhat faster w hen the I-125-activity was delivered with EGF possibly dependent on th e lower molecular weight allowing for faster diffusion. Furthermore, E GF-dextran associated I-125 seemed to penetrate somewhat faster after the EGF-receptors were blocked with non-radioactive EGF, probably due to the lack of binding preventing free diffusion. After administration of I-125-EGF-dextran or I-125-EGF, the binding patterns were superimp osed on the penetration patterns. In the penetration studies, the supe rimposed accumulations due to binding were removed by presaturation of the receptors with non-radioactive EGF. After a 1 hour incubation, bi nding of EGF-dextran associated I-125-activity could be seen only in a n outer region, with an approximative thickness of 50 mum, of the viab le cell layer. Extensive receptor specific binding in the deeper regio ns, at a depth of 100-200 mum, was seen after several hours incubation . In addition, low levels of non-specific binding in the central regio ns were seen when the I-125-activity was delivered with dextran withou t EGF. A similar low background binding was seen also in the centre Of spheroids incubated with I-125-EGF-dextran or I-125-EGF after saturat ion of the receptors with non-radioactive EGF. However, the major amou nt of radioactivity delivered as I-125-EGF-dextran or I-125-EGF had a receptor specific binding and, also in inner regions, it could be disp laced by non-radioactive EGF. Thus, EGF-dextran, which is a candidate compound for targeted therapy, allowed penetration of the applied radi oactivity and binding could be observed, after some hours, also in the inner regions of the spheroids.